In the past, as an example of these methods, a method of solidifying a powder material (for example, Japanese Unexamined Patent Application, First Publication No. S63-72802) was widely known in which a predetermined amount of water is added to metal powders or the like and then the whole mixture was mixed, charged into a mold equipped with a steam venting means, and subjected to a pressure molding under a low temperature of 100° C. or below in order to increase the apparent density at a low pressure when immobilizing theses kinds of powder materials. It was also widely known that a method of producing an alloyed aluminum sintered compact (for example, Japanese Unexamined Patent Application, First Publication No. S61-136602) in which a rapidly-solidified powder containing an alloyed aluminum is mixed with a lubricant powder having a melting point in the range of 100 to 300° C. to obtain a mixed powder. In this method, the mixed powder is heated at the temperature higher than the melting point of the lubricant powder and then pressured to obtain a powder compact. The powder compact is then sintered in order to obtain both a high-density powder compact and a sintered compact having smaller dimensional variation than the powder compact. Another known method is a method of producing a high-density sintered material in which raw powders such as steel powder or the like are heated under a temperature in the range of 350 to 650° C. in a non-oxidative atmosphere that does not impair the fluidity of the powders. The powders are charged into a mold coated with a lubricant pre-heated at 150 to 450° C., and subjected to compression and a warm molding to mold a powder compact. The powder compact is then heated and sintered in order to obtain a high-density sintered component on the basis that compressibility of the powders are abruptly improved at a temperature around 350° C. regardless of whether the powders are pure iron powders or alloyed steel powders.
In addition, a method of compression-molding a powder for powder metallurgy (Japanese Unexamined Patent Application, First Publication No. 2000-199002) has been widely known in which a powder for powder metallurgy having a lubricant incorporated therein is charged into a mold of which the surface of an inner wall is coated with the lubricant and then the powder is subjected to a compression molding under a warm or a hot atmosphere by setting the content of the lubricant in the powder for powder metallurgy as 0.20% by mass or less (0% by mass not being included in this range) per total mass of the powder in order to increase the molding density when an iron powder or an alloyed iron powder is subjected to the compression molding.
In the case of charging a raw powder for powder metallurgy into a mold and molding a compact by applying pressure under a warm atmosphere, in order to increase the fluidity of the raw powder when charging the raw powder into the mold and in order to increase the compressibility of the compact by increasing the lubricity between the raw powders and between the raw powder and the mold when molding the compact by applying pressure, a raw powder for a warm molding in which lithium stearate is mixed as a lubricant has been generally used as a raw material for powder metallurgy. However, in the case that lithium stearate is mixed in the raw powder, there is a problem in that fluidity of the raw powder is actually deteriorated when the raw powder is heated at 150 (C or higher even though the melting point of lithium stearate is approximately 220 (C. In addition, there is a problem in that sufficient lubricity and compressibility can not be obtained by using lithium stearate.
As disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-273502, it has been known that the fluidity of the raw powder is improved by adding a very small amount of fatty acid metallic salt having a small particle diameter, that is, the average particle diameter to the powder is 4 μm or less. However, there are weak points in that lubricity that can not be obtained by adding a very small amount of fatty acid metallic salt when the pressure molding is carried out, and fluidity is actually deteriorated when the amount thereof sufficient to obtain general lubricity is added. In addition, there is a problem in that the cost for producing fatty acid metallic salt having small particle diameter is more expensive than the cost for producing general fatty acid metallic salt, which is not economical.
As disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-294902, it has been known that a lubricant which contains a component having a low melting point that is lower than the temperature for the pressure molding is used. However, there is a problem in that fluidity of the raw powder can not be sufficiently obtained when the lubricant containing a lubricative component having a low melting point is heated up to the temperature for a warm molding.
For molding a powder compact as mentioned above, a powder for powder metallurgy having a solid lubricant incorporated in a raw powder thereof is charged into a mold by the use of a powder feeding device. The powder for powder metallurgy charged into the mold is then compacted, the powder compact is taken out of the mold, and the powder for powder metallurgy is charged again into the mold from which the powder compact was taken out. These steps are successively carried out to continuously mold the powder compact. The powder feeding device is equipped with a hopper and a feeder connected with a feeding pipe (for example, Japanese Unexamined Patent Application, First Publication No. 2003-191095).
As mentioned above and according to the related arts described in Japanese Unexamined Patent Application, First Publication No. S63-72802, Japanese Unexamined Patent Application, Japanese Unexamined Patent Application, First Publication No. S61-136602, Japanese Unexamined Patent Application, First Publication No. S58-71302, Japanese Unexamined Patent Application, First Publication No. 2000-199002, Japanese Unexamined Patent Application, First Publication No. 2000-273502, Japanese Unexamined Patent Application, First Publication No. 2001-294902, a warm molding is carried out by heating the powder for powder metallurgy before it is charged into the mold or heating the mold where the powder for powder metallurgy is charged into.